This invention relates to a method and apparatus for treating combustible gas streams containing hydrogen sulphide.
Hydrogen sulphide containing gas streams (sometimes referred to as "acid gas streams") are typically formed in oil refineries and natural gas processing units. Such streams should not be vented directly to the atmosphere because hydrogen sulphide is poisonous. A conventional method of treating a hydrogen sulphide-containing gas stream (which, if desired, has been pre-concentrated) is by the Claus process. In this process a part of the hydrogen sulphide content of the gas stream is subjected to combustion in a thermal stage taking the form of a furnace so as to form sulphur dioxide. The sulphur dioxide then reacts in the furnace with residual hydrogen sulphide so as to form sulphur vapour. The reaction between hydrogen sulphide and sulphur dioxide does not go to completion. The effluent gas stream from the furnace is cooled and sulphur is extracted, typically by condensation, from the cooled effluent gas stream. The resulting gas stream, still containing residual hydrogen sulphide and sulphur dioxide, passes through a train of stages in which catalysed reaction between the residual hydrogen sulphide and the residual sulphur dioxide takes place. Resulting sulphur vapour is extracted downstream of each stage. The effluent gas from the most downstream of the sulphur extractions may be incinerated or subjected to further treatment, eg by the SCOT or Beavon process, in order to form a gas stream which can be vented safely to the atmosphere.
Air may be used to support the combustion of hydrogen sulphide in the initial part of the process. The stoichiometry of the reactions that take place is such that relatively large volumes of nitrogen (which is, of course, present in the air that supports the combustion) flow through the process and therefore place a ceiling on the rate at which the gas stream containing hydrogen sulphide can be treated in a furnace of a given size. This ceiling can be raised by using commercially produced oxygen or oxygen-enriched air to support the combustion of the hydrogen sulphide.
Generally, depending on the concentration of the hydrogen sulphide containing gas stream, supply of commercially pure oxygen instead of air will result in the creation of excessive temperatures in the furnace which are liable to cause damage, particularly to the refractory lining of the furnace. Various methods are known for increasing the degree of enrichment of the air in oxygen without creating excessive temperatures. For example, United Kingdom Patent Application 2 173 780 A discloses moderating the temperature by introducing liquid water into the flame zone of the furnace. U.S. Pat. No. 5,352,433 discloses a particularly advantageous process in which the capacity or throughput of a Claus process is increased by conducting the combustion of the hydrogen sulphide in two separate furnaces. Accordingly, the overall amount of heat generated by the combustion is allocated between the two furnaces without the need to employ an external or recycled temperature moderator. Thus, a higher degree of uprating can be achieved than in other methods.
Typically, when combustion of the hydrogen sulphide takes place in two separate furnaces, it is possible to operate a process by retro-fitting an additional furnace and appropriate heat exchange equipment to an existing plant.
It is an aim of the present invention to provide a method and apparatus for recovering sulphur from hydrogen sulphide containing gas streams which is flexible to operate, which can be effectively controlled, and which can still offer at least some of the advantages of operation with commercially pure oxygen or oxygen-enriched air.